.TH FALLOCATE 1 "April 2014" "util-linux" "User Commands"
.SH NAME
fallocate \- preallocate or deallocate space to a file
.SH SYNOPSIS
.B fallocate
.RB [ \-c | \-p | \-z ]
.RB [ \-o
.IR offset ]
.B \-l
.IR length
.RB [ \-n ]
.I filename
.PP
.B fallocate \-d
.RB [ \-o
.IR offset ]
.RB [ \-l
.IR length ]
.I filename
.PP
.B fallocate \-x
.RB [ \-o
.IR offset ]
.RB \-l
.IR length
.I filename
.SH DESCRIPTION
.B fallocate
is used to manipulate the allocated disk space for a file, either to deallocate
or preallocate it.  For filesystems which support the fallocate system call,
preallocation is done quickly by allocating blocks and marking them as
uninitialized, requiring no IO to the data blocks.  This is much faster than
creating a file by filling it with zeroes.
.PP
The exit code returned by
.B fallocate
is 0 on success and 1 on failure.
.SH OPTIONS
The \fIlength\fR and \fIoffset\fR
arguments may be followed by the multiplicative suffixes KiB (=1024),
MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is
optional, e.g., "K" has the same meaning as "KiB") or the suffixes
KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB and YB.
.PP
The options \fB\-\-collapse\-range\fP, \fB\-\-dig\-holes\fP, \fB\-\-punch\-hole\fP and
\fB\-\-zero\-range\fP are mutually exclusive.
.TP
.BR \-c , " \-\-collapse\-range"
Removes a byte range from a file, without leaving a hole.  The byte range
to be collapsed starts at \fIoffset\fP and continues
for \fIlength\fR bytes.  At the completion of the operation, the contents of
the file starting at the location \fIoffset\fR+\fIlength\fR will be appended at the
location \fIoffset\fR, and the file will be \fIlength\fR bytes smaller.  The option
\fB\-\-keep\-size\fR may not be specified for the collapse-range operation.
.sp
Available since Linux 3.15 for ext4 (only for extent-based files) and XFS.
.TP
.BR \-d , " \-\-dig\-holes"
Detect and dig holes.  This makes the file sparse in-place, without using extra
disk space.  The minimum size of the hole depends on filesystem I/O block size
(usually 4096 bytes).  Also, when using this option, \fB\-\-keep\-size\fP is
implied.  If no range is specified by \fB\-\-offset\fP and \fB\-\-length\fP,
then the entire file is analyzed for holes.
.sp
You can think of this option as doing a "\fBcp --sparse\fP" and then renaming
the destination file to the original, without the need for extra disk space.
.sp
See \fB\-\-punch\-hole\fP for a list of supported filesystems.
.TP
.BR \-i , " \-\-insert\-range"
Insert a hole of \fIlength\fR bytes from \fIoffset\fR, shifting existing data.
.TP
.BR \-l , " \-\-length " \fIlength
Specifies the length of the range, in bytes.
.TP
.BR \-n , " \-\-keep\-size"
Do not modify the apparent length of the file.  This may effectively allocate
blocks past EOF, which can be removed with a truncate.
.TP
.BR \-o , " \-\-offset " \fIoffset
Specifies the beginning offset of the range, in bytes.
.TP
.BR \-p , " \-\-punch\-hole"
Deallocates space (i.e., creates a hole) in the byte range starting at
\fIoffset\fP and continuing for \fIlength\fR bytes.  Within the
specified range, partial filesystem blocks are zeroed, and whole
filesystem blocks are removed from the file.  After a successful
call, subsequent reads from this range will return zeroes.  This option
may not be specified at the same time as  the \fB\-\-zero\-range\fP option.
Also, when using this option, \fB\-\-keep\-size\fP is implied.
.sp
Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0),
Btrfs (since Linux 3.7) and tmpfs (since Linux 3.5).
.TP
.BR \-v , " \-\-verbose"
Enable verbose mode.
.TP
.BR \-x , " \-\-posix"
Enable POSIX operation mode. In that mode allocation operation always completes,
but it may take longer time when fast allocation is not supported by the underlying filesystem.
.TP
.BR \-z , " \-\-zero\-range"
Zeroes space in the byte range starting at \fIoffset\fP and
continuing for \fIlength\fR bytes.  Within the specified range, blocks are
preallocated for the regions that span the holes in the file.  After
a successful call, subsequent reads from this range will return zeroes.
.sp
Zeroing is done within the filesystem preferably by converting the
range into unwritten extents.  This approach means that the specified
range will not be physically zeroed out on the device (except for
partial blocks at the either end of the range), and I/O is
(otherwise) required only to update metadata.
.sp
Option \fB\-\-keep\-size\fP can be specified to prevent file length
modification.
.sp
Available since Linux 3.14 for ext4 (only for extent-based files) and XFS.
.TP
.BR \-V , " \-\-version"
Display version information and exit.
.TP
.BR \-h , " \-\-help"
Display help text and exit.
.SH AUTHORS
.UR sandeen@redhat.com
Eric Sandeen
.UE
.br
.UR kzak@redhat.com
Karel Zak
.UE
.SH SEE ALSO
.BR truncate (1),
.BR fallocate (2),
.BR posix_fallocate (3)
.SH AVAILABILITY
The fallocate command is part of the util-linux package and is available from
.UR https://\:www.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
Linux Kernel Archive
.UE .
